The present invention concerns a process and an apparatus for controlling the output of an air-breathing fuel cell system comprising an air supply line, a variable-speed compressor operatively located in the air supply line leading to the fuel cell, a variable absorption capacity expander operatively located in the air discharge line, and a common shaft operatively associating the compressor and the expander.
German Patent No. 4,021,097 describes a fuel cell system in which the process air is compressed by one or more compressors before entering the fuel cell. After flowing through the fuel cell, the discharged exhaust air is expanded by a turbine so that the energy can be recovered. The turbine, the compressor, and an additional drive motor are positioned on a common shaft. The disadvantage of this known system is that neither the air volume flow nor the working pressure of the process air can be changed independently of each other.
An object of the present invention is to provide a process and apparatus for optimizing the output of the fuel cell system in all operating ranges while the energy required for compressing the air is simultaneously minimized.
This object has been achieved according to the present invention by providing an expander with variable absorption capacity, with the rotary speed of the compressor and the absorption capacity of the expander being preset to desired values by a control device.
By coupling the speed-variable compressor with the variable absorption capacity expander, the fuel cell system can be operated at all times under optimum conditions and, at the same time, the energy required for compressing the air can be reduced. An optimum output of the fuel cell system is achieved when the excess air and the working pressure are at all times optimally adjusted to the operating conditions and to the given system configuration. The air volume flow can be controlled by varying the rotary speed of the compressor. Thus, because of the common shaft, the rotary speed of the compressor also determines the rotary speed of the expander. However, it is also possible to control the working pressure by way of the variable absorption capacity of the expander.
A further advantage is obtained by the fact that the residual energy contained in the waste air is converted by the expander into mechanical energy and returned to the compressor via the common shaft. Thus, the energy required by the compressor is reduced.